he FBI was alerted by Russia's security services to serious new concerns about one of the Boston bomb suspects as recently as last November

Boston bomber: FBI 'dropped the ball' over Tamerlan Tsarnaev

The FBI was alerted by Russia's security services to serious new concerns about one of the Boston bomb suspects as recently as last November, it was claimed on Sunday night.

Boston marathon bomber Tamerlan Tsarnaev’s application for US citizenship was reportedly rejected after US officials carried out a background check that showed he had been interviewed in 2011 by the FBI.Photo: GETTY IMAGES

As the agency was accused of "dropping the ball" over the case, NBC News reported that Tamerlan Tsarnaev had been seen making six visits to a known Islamic militant in a mosque in the Russian republic of Dagestan.

The visits came during a six month trip that Tamerlan made to the city of Makhachkala to see his family, NBC said.

According to a local police official, a case file on Tsarnaev was then handed over to the FBI along with a request for further information. However, the FBI never replied.

The agency has already admitted that it interviewed Tamerlan Tsarnaev in 2011 after Russia raised concerns that he was becoming a follower of radical Islam, but found nothing "derogatory" against him and did not pursue the case further.